The present invention relates to a wire discharge machining power source, wherein in wire-cut discharge machining a voltage having a polarity opposite to that of a voltage applied during electric discharge machining, is applied between a wire electrode and a workpiece to set an average machining voltage to zero, thereby preventing electrolytic effect and hence galvanic corrosion.
In electric discharge machining using water as a working fluid, an electrolytic effect occurs. In particular, when a workpiece such as a carbide, which is easily subject to galvanic corrosion, is machined roughly, the workpiece tends to be embrittled, resulting in inconvenience. In second cutting, the surface of the workpiece is roughened by selective galvanic corrosion of particles constituting the workpiece due to the electrolytic decomposition. A finished surface is roughened, thus resulting in inconvenience. In order to prevent this, a technique is known to those skilled in the art wherein a voltage having a polarity opposite to that of a voltage applied during machining is applied between an electrode and a workpiece during nonmachining, and an average voltage applied throughout machining and nonmachining periods is set to zero to prevent the electrolytic effect.
A reverse voltage applied between the workpiece and the electrode may vary in accordance with a change in thickness of the workpiece and a leakage current flowing from the gap between the workpiece and the electrode and the like. When a peak value of the reverse voltage is too large, the electrode is greatly damaged. When positive discharge is performed with small energy, damage to the workpiece which is caused by reverse voltage discharge worsens the roughness of the machined surface. However, when a peak value of the reverse voltage is too small, the time for applying the reverse voltage must be prolonged, thus degrading machining efficiency. In order to solve the above problem encountered in electric discharge machining of the type wherein the zero average voltage is obtained by application of the reverse voltage, a reverse voltage peak value must be controlled.